The invention described herein relates to technology for splicing medium and high-voltage electrical cables. In particular, the invention is a pre-assembled electrical splice component for splicing medium and high-voltage electrical cables.
Medium and high-voltage electrical cables typically consist of a central copper or aluminum conductor surrounded by several concentric layers of various materials. Specifically, the central conductor is typically surrounded by an insulative layer, which in turn is surrounded by a conductive shield layer and an outer protective jacket. The material and the dimensions of the insulative layer are chosen so that the required insulation of the central conductor is achieved. The shield layer is a conductive layer which electrically is kept at ground level and which is typically designed so that it can withstand short circuit currents, in the event a short circuit should occur. The conductive shield is usually comprised of a layer of shield wires. The outer jacket provides the cable with protection against mechanical impacts, and prevents the ingress of humidity or water into the splice.
When building up a power distribution network, it is often necessary to splice the ends of medium or high-voltage cables together. The splicing is done in a way experienced over many decades. First, lengths of each of the different cable layers are removed adjacent the ends to be spliced, such that a portion of each layer is exposed. Next, the central conductors are connected through the use of known screw-type or crimped connectors. Finally, it is necessary to reconstruct the electrical configuration of the original cable over the area of the splice. That is, the splice must be provided with an insulation layer over the central conductor, a conductive shield and an outer protective layer.
Currently, the insulation layer, conductive shield and protective jacket are typically reconstructed by individually installing multiple layers of material onto the splice, or by using multi-layer components which incorporate more than one layer into a single sleeve for simultaneous installation over the splice. These sleeves are preferably formed from elastomeric materials which are mechanically expanded and placed onto a support core. The expanded sleeve can then be moved over one cable end prior to the splicing, and then easily moved back over the splice for installation. The support core is then removed after placing the sleeve over the completed splice, so that the sleeve elastomerically shrinks down onto the splice.
The support core may take any of several forms. For example, support cores can consist of tubes which are pulled out of pre-stretched components and which can then be split into parts for removal from the cable. Such an arrangement is shown, for example, in FIG. 2 of the German Offenlegungsschrift DE 30 01 158 (S.A. des Cableries). An alternative approach can be seen from the European Patent Application EP 0 702 444 (Fournier). Yet another type of support core is a spirally wound system which is gradually removed, thereby allowing the system to shrink down as described in U.S. Pat. No. 3,515,798 (3M) or the European Patent EP 0 101 472 (3M) or the German Offentegungsschrift DE 37 15 915 (3M) or the U.S. Pat. No. 5,589,667 (3M).
It is known to provide an elastomeric multi-layer sleeve which may, for example, reconstruct the insulation layer and the conductive shield layer. It is also known to include in the sleeve additional stress control components for smoothing areas of electrical stress which unavoidably occur at points of discontinuity in the conductive shield.
After installing the elastomeric multi-layer sleeve (comprising an insulation layer and a conductive layer) over the splice, the portion of the sleeve which provides a conductive shield must be electrically connected to the exposed conductive shield layers of the two cable ends. This is normally achieved by placing an additional conductive layer over the entire multi-layer sleeve. The additional conductive layer is typically formed from, for example, a metallic braid which extends over the entire multi-layer sleeve, as well as over the exposed portions of the conductive shield layers at both cable ends. Most preferably copper braids are used for this purpose. For a reliable connection, spring connectors such as constant force springs are applied onto the conductive braid at the points where the conductive braid overlaps and contacts the exposed portions of the cable shield layers. The constant force springs have the advantage of establishing a secure and permanent electrical contact between the conductive braid and the shield of the cable which is capable of withstanding the electrical currents which would result from a short circuit. The use of constant force springs is important, because experiments have shown that a sufficient electrical contact is not provided between the conductive braid and the cable shield by, for example, an elastomeric rubber sleeve placed over the braid, especially in case of short circuits.
Finally an outer protective jacket must be placed over the entire splice, and thereby serves as a cable jacket replacement. The outer jacket is typically installed by using a pre-stretched tube which is also placed over to one of the cable ends prior to the establishment of the splice. As a final act this protective outer layer is shrunk over the entire connection. The outer jacket is selected so that it provides adequate sealing forces to prevent moisture from entering the splice.
The multi-layer sleeve that is directly shrunk over the splice in itself already represents a progress over the conventional technology, as it permits several components to be combined into a single unit, such as the above-mentioned stress control components, an electrode providing a Faraday cage over the actual splice, the insulation layer and the conductive shield layer. Thus, the above described method already significantly reduces the number of components to be applied in order to establish a complete splice of medium or high-voltage cables.
There have been attempts to further simplify the manner in which medium and high-voltage cables are spliced. As a next step, configurations have been developed in which the conductive braid and the cable jacket replacement are also placed onto the multi-layer component to further reduce the number of components which must be individually applied when forming a splice. However, even when the conductive braid and protective jacket are incorporated onto a single support core for application to a splice, it is still necessary to establish a good and permanent contact between the braid and the shield layer of the cables by applying the constant force springs. This is an additional step which must be performed, and it would be desirable if it could be eliminated. However, removing the step is difficult, as constant force springs are needed for reliable contact, but their application is not easily accomplished, as will be described below.
The splicing method described above can be understood, for example, from the figures of German Offenlegungsschrift DE 30 27 097 (Siemens). FIG. 1 of the reference shows a cross-sectional view through a portion of a splice prior to shrinking down the components, and illustrates many of the parts required to complete the splice. The cable conductor 3 is surrounded by the cable insulation 4 which is coated with a shield 5, additional shield wires 6 and the cable jacket 7. The conductor is crimped with a connector 2 to the other cable end (not shown). A multi-layer sleeve 10 includes an inner electrode 12 serving as a Faraday cage, an insulation layer 11 and an outer conductive shield 15, 16 placed onto a support core 17 which consists of a spiral that can be removed so that the entire sleeve would shrink down onto the connection. Furthermore, an additional conductive shield 18 and a second support core 21 are shown onto which the outer cable jacket replacement 20 is placed.
FIG. 1 of DE 30 27 097, however, does not represent any means for good and permanent electrical contact between the shields 5, 6 of the cable and the shields 15, 16, 18 of the splice. Accordingly, constant force springs are not depicted.
Constant force springs, however, can be best understood from the U.S. Pat. No. 5,028,742 (3M) which particularly well depicts the use of a constant force spring. In FIG. 3 of the reference, a typically prepared cable end is shown with a conductor 14, insulation layer 16, a conductive shield 18, and a protective jacket 20. In this particular reference the actual cable connection is established by means other than that described above. However, it can be seen that a conductive braid is used in the form of a strip 26, 28 which is mounted onto the shield 18 of the cable through the use of the constant force spring 24. The complete connection can be best seen in FIG. 5 which primarily shows the proper use of the constant force springs 24, although the connection as such is different from the connection depicted in FIG. 1 of DE 30 27 097.
The multi-layer sleeve which is placed onto the splice connection can have a variety of forms. An example of a multi-layer sleeve is described in EP 0 435 569 (3M). On a support core 20 is placed a molded multi-layer body 10 having an inner electrode 18 to establish a Faraday cage over the crimp or screw-type connector, two stress control components 14, 16, an insulation layer 12 and an outer conductive shield 11. This component is placed onto the cable connection, and then is further covered with other components (such as a conductive braid, constant force springs, and a projective jacket) as described above.
Another multi-layer sleeve configuration is described in WO 95/31845 (3M) which differs from the preceding one by the absence of stress control components. This is possible as the insulating material itself provides the stress control.
A more compact system is described in EP 0 393 495 (Pirelli) and especially in FIGS. 7 and 9. The reference shows a support core 19 onto which a multi-layer sleeve 3' is placed. Also placed onto core 19 is a cable jacket replacement 21. Due to the longer length of cable jacket replacement 21, the jacket replacement 21 is folded back upon itself at both ends. Between the multi-layer sleeve 3' and the cable jacket replacement 21 a conductive layer, typically a braid 16, is placed.
The installation method for a splice assembly similar to that shown in EP 0 393 495 is, for example, described in the instruction manual from Alcatel/Euromold for the cold shrink splice 24CSJ. Although the instruction manual is in German, the assembly method can be clearly understood from the figures. Pages 1-3 of the instructions show the preparation of the cable ends. Especially on page 2 it can be seen in which way the shield wires (Schirmdrahte) are exposed and folded back over the outer cable jacket. On page 5 it is depicted in which way the complete splice body (Muffengehause) is moved over one cable end (to the right) and an additional body for portions of the cable jacket replacement is moved over the other end (to the left). The connection of the central conductors is established as depicted on page 6 and in this case the Faraday cage is established by a different method as depicted on page 7. The splice body is moved over the connection as shown on page 8 and the support core (which in this case consists of two tubes) is removed so that the entire system shrinks down as depicted in pages 9 and 10. Page 11 shows the decisive feature insofar as it can be seen that the braid has to be removed by folding it back, and the constant force spring (Rollfeder) is partially applied onto the conductive shield wires of the cable ends. To do this, the constant force spring which is supplied as a separate component is opened at the outer end and essentially a single turn is applied onto the cable by partially unwinding the constant force spring. Then, as depicted in the lower figure of page 11, the copper braid is folded back onto this single layer of the constant force spring as well as onto the cable. Then, the constant force spring as such is further unrolled over the braid so that at the end the braid is pressed onto the cable for secure electrical contact. It must be understood that to achieve this result portions of the braid have to be opened so that the upper layers of the constant force springs can be placed on top of the copper braid. Therefore, at the end of the process, the following configuration is obtained: The cable with the folded back cable shield wires onto which approximately a single layer of the constant force spring is applied, this single layer being covered by the copper braid which has to be opened on the point of the transition, the entire configuration then followed by the additional layers of the constant force spring which are applied onto the copper braid. As a final step, as depicted on page 12, the cable jacket replacement portions are applied onto the two ends of the spice, thereby covering the previously exposed portions of the braid.
As can be seen from the above description, application of the constant force spring is typically rather difficult and cumbersome, even when the conductive braid is applied as a part of the multi-layer sleeve. Therefor, it is desired to be able to apply all of the components, including the constant force springs, in a single action.